Thanks to Toby for alerting me to the fact that it appears that the UK will be getting the Season 4 DVD before those of you in the US. The dates and links below are the latest dates we have currently. (Note: It appears that the UK no longer has an Amazon page for the Season 4 Blu-Ray).
The Season 4 DVD and Blu-Ray Releases for Season 4 are now both available for pre-ordering.
USA Version - DVD : 9th December 2008
USA Version - Blu-Ray : 9th December 2008
UK Version - DVD : 27th October 2008
Canadian Version - DVD : 9th December 2008
Canadian Version - Blu-Ray : 9th December 2008